The present invention relates generally to an electric double layer capacitor, and more particularly to a high performance double capacitor comprising aluminum carbon composite electrodes and a high performance electrolyte solution, and further includes a method for fabricating the same.
Double layer capacitors are energy storage devices that are able to store more energy per weight than traditional capacitors and they can typically deliver the energy at a higher power rating than many rechargeable batteries. Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolytic solution. This allows ionic current to flow between the electrodes while preventing electrical current from shorting the cell. On the back of each of these active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
When electric potential is applied across the two electrodes in a double layer capacitor, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface the charge is absorbed into the solid liquid interface region. This is accomplished by absorption of the charge species themselves or by realignment of the dipoles of the solvent molecule. The absorbed charge is held in the region by the opposite charges in the solid electrode.
The use of carbon electrodes in electrochemical capacitors represent a significant advantage in this technology because carbon has a low atomic weight and carbon electrodes can be fabricated with very high surface areas. Fabrication of double layer capacitors with carbon electrodes has been known in the art for quite some time, as evidenced by U.S. Pat. Nos. 2,800,616 (Becker), and 3,648,126 (Boos et al.).
A major problem in many carbon electrode capacitors is that the performance is often limited because of the high internal resistance of most carbon electrodes. This high internal resistance is due primarily to the high contact resistance of the carbon--carbon contacts. This high resistance translates to large ohmic losses in the capacitor during the discharge phase. Lowering this internal resistance in double layer capacitors is achieved primarily through the reduction of electronic resistance in the electrode.
It is also difficult to simultaneously achieve a combination of large surface area and sufficient control of the carbon electrode porosity. Carbon electrode porosity translates to the degree of electrolyte accessibility to the surface carbon atoms.
In order to increase the operating voltage of many double layer capacitors, individual cells are often stacked in series. Current paths between cells must be minimized in order to reduce ohmic losses. The optimum design is to have adjacent cells separated with only a single current collecting plate. This plate must be non-porous so that no electrolytic solution is shared between cells. Separation prevents losses due to the shunt current path between cells. This type of design is called bipolar. In a bipolar double layer capacitor, one side acts as a positive electrode and the other side acts as a negative electrode for an adjacent cell. U.S. Pat. No. 3,536,963, issued to D. L. Boos, is an example of this bipolar double layer capacitor.
Another double layer capacitor design that has recently become popular is a spiral wound cell. U.S. Pat. No. 5,150,283, issued to Yoshida, et al., discloses one example of the spiral wound cell. In the Yoshida et al. patent an electric double layer capacitor has a pair of polarizable electrodes composed of electrically conductive substrates. The substrates are coated in layers of a mixture of activated carbon with a water soluble material based binding agent. The electrodes face each other interposed by a separator impregnated with an electrolyte. This capacitor has advantageous features of both conventional capacitors which use aqueous electrolytes and capacitors which typically use organic solvent electrolytes.
An advantage of the spiral wound cell of the double layer capacitor is that large surface area electrodes can be rolled into a small case. The large electrodes greatly reduce the internal resistance of the capacitor and the case greatly simplifies the capacitor seal or the sealing required for the double layer capacitor. In a bipolar design each cell must be sealed around the perimeter of the electrode. However, in a wound design, only the outer can requires sealing. This design is not as efficient as a bipolar design when the cells are stacked in series, because the resistance of the lead wires will add to the ohmic losses.
The present invention, however, is more concerned with electrochemical or double layer capacitors having aluminum/carbon composite electrodes. The metal/carbon composite electrodes, and more particularly, the aluminum/carbon composite electrodes, tend to minimize the internal resistance of the electrode. Teachings in the related art that are of particular importance involve methods of fabricating the aluminum/carbon composite electrodes, methods of fabricating and adhering the current collector to the composite electrode, and suitable electrolytes that can be used with high performance double layer capacitors.
Various fabrication techniques for reducing the internal resistance of carbon composite electrodes have been disclosed over the recent years. For example, the Yoshida et al. patent (U.S. Pat. No. 5,150,283) discloses a method of fabricating a aluminum/carbon composite electrode by depositing carbon powder and other electrical conductivity-improving agents on an aluminum substrate.
Another related approach is disclosed in U.S. Pat. No. 4,597,028 (Yoshida et al.) which teaches that the incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished through weaving metallic fibers into carbon fiber preforms.
U.S. Pat. No. 4,562,511, (Nishino et al.) describes yet another approach where the carbon fiber is dipped into an aqueous solution such that a layer of a conductive metal oxide, and preferably a transition metal oxide, is formed in the pores of the carbon fibers. Nishino et al. also discloses the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition.
Still another related method is disclosed in U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963 (Tatarchuk et al.). These disclosures demonstrate that metal fibers can be interwoven with the carbon preform and sintered to create a structurally stable conductive matrix which may be used as a composite electrode. The Tatarchuk et al. patents also teach a process that reduces the electrical resistance in the composite electrode by reducing the number of carbon--carbon contacts, which current must flow through to reach the metal conductor. This approach works well if stainless steel or nickel fibers are used as the metal. However, this approach has not been successful when aluminum fibers are used because of the formation of aluminum carbide during the sintering or heating of the composite electrode.
The use of aluminum in fabrication processes of double layer capacitors is important because aluminum is the optimum metal in terms of cost, availability and performance. For example, with an aluminum/carbon composite electrode, in a double layer capacitor with a nonaqueous electrolyte, it is quite possible to achieve an operating voltage of 3.0 volts. However, with nickel or stainless steel in lieu of aluminum, the operating voltage must be reduced to less than 2.0 volts.
Related designs of double layer capacitors are also discussed in U.S. Pat. No. 4,438,481, issued to Phillips, et al.; U.S. Pat. No. 4,597,028 issued to Yoshida, et al.; U.S. Pat. No. 4,709,303 issued to Fujiwara, et al.; U.S. Pat. No. 4,725,927, issued to Morimoto; and U.S. Pat. No. 5,136,472, issued to Tsuchiya, et al.
Another area of great concern in the fabrication of double layer capacitors is concerned with the method of fabricating the current collector plate and adhering the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor.
The Nishino et al. patent (U.S. Pat. No. 4,562,511) suggests plasma spraying of molten metals such as aluminum onto one side of the polarizable electrode thereby forming an appropriate layer which acts as the current collector. This patent further considers alternative techniques for bonding and/or forming the current collector including arc-spraying, vacuum deposition, sputtering, non-electrolytic plating, and use of conductive paints.
The Tatarchuk et al. patents (U.S. Pat. Nos. 5,102,745, 5,304,330, and 5,080,963) show the bonding of a metal foil current collector to the electrode by sinter bonding the metal foil to the electrode element.
U.S. Pat. No. 5,142,451 (Kurabayashi et al.) discloses a method of bonding of the current collector to the electrode by a hot curing process such that the material of the current collectors enter the pores of the electrode elements.
U.S. Pat. No. 5,099,398 (Kurabayashi et al.) discloses a method of bonding of the current collector to the electrode by chemically bonding a thin film collector such that some of the material of the current collectors enter the pores of the electrode elements. This patent further discloses some other conventional methods of bonding the current collector to the electrode including the use of electrically conducting adhesives and bonding under pressure and heat.
Still other related art concerned with the method of fabricating and adhering current collector plates can be found in U.S. Pat. Nos. 5,065,286; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued to Kurabayashi et al.
Accordingly, there is a continuing need for improved double layer capacitors with carbon/aluminum composite electrodes. These improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and power density ratings. These improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage.
Furthermore, it is also apparent that improvements are needed in the techniques and methods of fabricating double layer capacitors with aluminum/carbon composite electrodes in an effort to lower the internal resistance of the double layer capacitor and maximize the operating voltage. Since capacitor energy density increases with the square of the operating voltage, higher operating voltages translate into higher performing capacitors because of the significantly higher energy densities and power output ratings.